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Heart of a sunspot revealed

By Eugenie Samuel

The heart of a sunspot has been mapped for the first time, by researchers working with the SOHO satellite that monitors solar activity.

Dark red sunspots disappear and reappear on the surface of the Sun over an 11-year cycle. Researchers study the spots because their appearance seems to accompany magnetic storms in the inner solar system, but they do not yet understand what kinds of solar currents are involved.

But this may change now that physicist Philip Scherrer and his colleagues at Stanford University have mapped the flow of material under a sunspot. The Stanford group is the first to process SOHO data to reveal local seismic waves, which shake only a part of the Sun’s surface, rather than the entire star.

As well as being a technical achievement, their map solves a puzzle, says Douglas Gough, who studies solar physics at Cambridge University. When observed, sunspots do not fly apart, despite the fact that material appears to flow outwards from them.

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“But underneath, the flow is coming inwards and holding the spot together,” Gough says. “There’s a very thin layer on the surface where the flow turns round.”

The Stanford group used data from SOHO’s Michelson Doppler Imager (MDI) which looks at deep red light emitted by nickel atoms on the surface of the Sun. The frequency of the emitted light is shifted by an amount depending on the velocity of the nickel atom – an effect called the Doppler shift.

To make the map, the team compared pairs of points that were moving in the same way, but at different times. The match told them that a seismic wave had moved between the points.

As the speed of sound is higher deeper in the Sun where it is more dense, the speed of the wave between the points told them how deep it was, and its direction told them which way the current was flowing. “We had this method in June 1998 but then we lost SOHO for six months and only now were we able to confirm it,” says Scherrer.

Gough says that once researchers have a map of subsurface flows over the whole Sun, they can use it to probe the very centre, where the nuclear reactions are taking place. “From the point of view of understanding how the Sun really operates, that’s extremely important,” he says.